1. Field of the Invention
The present invention relates to the treatment of aluminum/lithium alloys which are suitable for aerospace air frame applications, and more particularly to an improved lithium-containing aluminum base alloy product obtained by eliminating anisotropic behavior (i.e., directional properties), and a method of producing the same.
2. Background of the Invention
In the aerospace industry, It has been generally recognized that one of the most effective ways to reduce the weight of a craft is to reduce the density of the materials used in its construction, while maintaining the maximum mechanical property characteristics.
The materials most typically contributing to the weight problem are metallic materials insofar as they fall to offer the strength-to-density and stiffness-to-density advantages that are obtainable in some of the advanced composite material systems.
For purposes of reducing aluminum alloy densities up to 20%, lithium additions have been made. It is known that such aluminum-lithium alloys can present high strength and stiffness, and exhibit good corrosion-resistance, and improved crack growth resistance, properties.
However, the addition of lithium to aluminum alloys is not without problems. For example, previous alloys of this kind have, in the past and in comparison to other aircraft alloys, suffered from a reduction In such properties as fracture toughness and decreased ductility, with associated problems in ingot casting and subsequent working. Where the use is for aircraft parts, it is imperative that the lithium-containing alloy have both improved fracture toughness and strength properties.
It is well known that mechanical deformation, by such conventional milling processes as hot and cold rolling, can lead to the development of a metallurgical condition known as "texturing", a phenomenom brought about by crystollographic preferred orientations In sheet or strip forms of metallic materials. These conditions are manifested as anisotropic material properties which can be detrimental to the mechanical properties and structural behavior of the product. For example, anisotropy of mechanical properties can result in an appreciable variation of the strength and ductility of the product depending on the direction within the plane of the sheet or strip in which the properties are measured.